Excimer lasers are typically configured with two opposing mirrors that define a resonant cavity, and a gain medium placed therebetween. The gain medium is a gas mixture of a halogen gas and a rare gas that is excited using electrodes to generate an intracavity laser beam that oscillates between the mirrors. Typical excimer laser systems cyclically activate the electrodes to generate a pulsed intracavity laser beam. One of the mirrors is partially transmissive to produce an output laser beam.
Excimer lasers are frequently equipped with stable resonators. A stable resonator is one in which the intracavity laser beam is focused into a confined path, typically with curved resonator mirrors and/or intracavity lenses. In excimer lasers and some other laser types, a nearly stable cavity can be formed by two flat mirrors since the number of round trips that light makes in this type of cavity is relatively small. In a stable resonator, the pulses bounce back and forth between the mirrors in a narrow intracavity beam without suffering significant diffraction losses due to energy loss caused by beam divergence.
Additionally, unstable resonators have been developed, in which a significant portion of the intracavity beam leaves the resonator on each round trip due to expansion of the beam caused by curved intra-cavity elements. As pulses bounce back and forth between the mirrors, the pulses diverge and spread out. A primary advantage of an unstable resonator is that it allows for high energy extraction efficiency (especially when using a large volume gain medium) with low divergence.
There are, however, drawbacks to unstable resonators. First, narrow linewidths from unstable resonators are difficult to achieve. Intracavity wavelength selectors, such as gratings and prisms, work only with an intracavity beam that is retro-reflected onto itself. The intracavity beam in a typical unstable resonator does not retro-reflect onto itself (meaning that a collimated beam is reflected as an uncollimated beam, and vice versa), which precludes using such wavelength selectors in an unstable resonator.
Secondly, the output beam from an unstable resonator typically contains significant low coherence light which is caused by Amplified Spontaneous Emission (ASE). Low coherent light in the output beam caused by ASE is highly divergent since it has not been subject to expansion by the curved A intra-cavity mirrors and/or lenses. The low coherence light reduces the spatial coherence of the output beam. There are applications, such as Fiber Bragg Grating (FBG) production, which require an output beam having low divergence and a narrow linewidth, and are incompatible with unstable a resonators that produce significant low coherence light.
There is a need for laser system cavity design that produces a low divergence, narrow line-width output beam, which still utilizes an unstable resonator for high energy extraction efficiency. Further, such a laser system design should also minimize low coherence light in the output beam caused by ASE.